Your nose is not a spare mouth
A journalist plugged his nostrils for ten days to prove a point about breathing, and the point proved him wrong.
James Nestor wanted to know how much breathing route actually matters, so he had a Stanford lab seal his nose shut with silicone plugs for ten days, forcing him to breathe only through his mouth. He expected mild discomfort. Instead his snoring went from a few minutes a night to hours, his blood pressure climbed, and he developed sleep apnea he'd never had before.
Then came the second ten days: nose only, mouth taped shut at night. The apnea vanished. Snoring stopped. His resting heart rate variability, a marker of how well the nervous system recovers, improved sharply. Nothing else in his life had changed except which hole the air went through.
The nose isn't a passive intake valve. It filters, humidifies, pressurizes air, and releases nitric oxide the mouth doesn't, which helps the lungs absorb oxygen more efficiently. Nestor's book Breath turns this into an argument that modern humans have quietly become chronic mouth-breathers, and are paying for it in sleep no one traces back to how they inhale.
Worth noticing, before you diagnose yourself: which one you're doing right now.